The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
Typically, a spiral wound membrane element is made by wrapping one or more membrane leaves around a perforated central tube. One edge of a feed carrier sheet is placed in a fold of a generally rectangular membrane sheet. The fold of the membrane sheet is positioned along a perforated central tube. A permeate carrier sheet is provided between each pair of membrane sheets. Glue lines seal the permeate carrier sheet between adjacent membrane sheets along three edges, forming a membrane leaf. The fourth edge of the leaf is open to the perforated central tube. All of the sheets are wrapped around the perforated central tube.
In use, the spiral wound membrane element is housed in a pressure housing, also referred to as a pressure tube or a pressure vessel. A pressurized feedstock is delivered at an upstream end of the pressure housing and flows into the spiral wound membrane element. Within the spiral wound membrane element, the pressurized feedstock flows through the feed spacer sheets and across the surface of the membrane sheets. The membrane sheets may have a discriminating layer that is suitably sized for microfiltration, ultrafiltration, reverse osmosis or nanofiltration. A portion of the pressurized feedstock is driven through the discriminating layer by transmembrane pressure to produce a permeate stream. The permeate stream flows along the permeate carrier sheets into the central tube for collection outside the pressure housing. The components of the pressurized feedstock that do not pass through the membrane, also referred to as retentate, continue to move through the feed spacer sheets to be collected at a downstream end of the pressure housing.
Some specific industries (for example the dairy industry) require sanitary spiral wound membrane elements that meet the requirements of the Sanitary 3A Standards for Crossflow Membrane Modules. Sanitary problems can arise in areas of low flow, also referred to as areas of tight tolerance. In areas of tight tolerance, there is limited fluid access and therefore limited flushing to remove solids or provide sanitization solutions. One region that typically has tight tolerance is between an inner surface of the pressure housing and the outer surface of the spiral wound membrane element, referred to as the annular space.
A common solution to low flow in the annular space is to direct a portion of the feedstock flow into the annular space. This is referred to as bypass flow. Bypass flow improves flushing of the annular space; however, the bypass flow also reduces the volume of feedstock that passes through the spiral wound membrane element to contribute to the production of permeate.
Various factors affect permeate production including temperature, osmotic pressure gradients, polarization layer, the charge of materials, fouling and the balance of fluid pressures across the membrane sheets, referred to as transmembrane pressure. The pressure of the feedstock within the feed spacer sheets influences the transmembrane pressure. As the permeate volume increases, the pressure and velocity of the feedstock within the feed spacer sheets decreases. Furthermore, the flow of feedstock through the feed spacer sheets is exposed to resistance, which is a source of head loss. Due to the volume loss of the feedstock and the head loss, the pressure and velocity of the feedstock within the feed spacer sheet decreases along the length of the spiral wound membrane element. This decreased feed spacer sheet pressure decreases the transmembrane pressure and decreases overall permeate production. The decreased velocity reduces disruption of the polarization layer at the membrane surface, which further reduces permeate production.
Typically, more than one spiral wound membrane element is housed in one pressure housing. For example, in the dairy industry between one and ten spiral wound membrane elements can be housed in one pressure housing. The multiple spiral wound membrane elements are connected in series and they typically share a common central tube. A standard dairy feedstock is introduced into the upstream end of the pressure housing at a pressure of about 100 psi. Along the length of a given spiral wound membrane element, the feed spacer sheet pressure may decrease about 5 to 10 psi. This pressure decrease can accumulate when multiple spiral wound membrane elements are used in one pressure housing and decrease the production of permeate within a given pressure housing.